What if you could make a slight change to one of your e-mails and get a 20 percent lift in opens or a 10 percent increase in sales? You would do it in an instant. But how do you know which element to change? That's where testing comes in.

E-mail marketing makes it easy to quickly test important elements of your e-mail at very little or no extra cost. With testing, you can find out what factors influence the success of your e-mail. Follow these five steps to create an effective and measurable test.

1. Decide what to test.
Because testing with e-mail is so easy, it's tempting to test many elements at once. Start by testing just one. Why? If you test more than one element in the same e-mail, it's challenging--and sometimes impossible--to determine what influenced the response. Here are some easy and telling tests:

Subject lines--
Create two different subject lines for the same e-mail communication. For example, a boutique owner just added a home and garden section and wants to get the word out to her customers. Here are the subject lines she'll test.

Subject line No. 1: New! Home and garden section added

Subject line No. 2: Get what you need for your home and garden

Long vs. short copy-- Is less really more? Create a shorter version of your current newsletter with teasers and links to your website. Or create two versions of a promotional e-mail. Keep one very short and to-the-point and make the other a little longer by adding additional, useful information.

Special offers--
Create two different offers. For example, an online bookseller wants to get rid of last season's bestsellers. He sends the following offers to see which one gets a better response.

Offer No. 1: Buy three books and get one free

Offer No. 2: Buy three books and get free shipping

Other tests could include the time of day or day of the week you send, whether you include an image, types of calls-to-action, and the placement of a call-to-action button or link. I'm sure that you will come up with other areas you would like to test as well.

2. Decide how to measure success.
What will you measure to determine success? Possibilities include website traffic, response to an offer, sales, opens and click-throughs. Whichever you decide on, be confident that you can attribute an increase--or decrease--in the area you measure directly to the e-mail you send. The easiest place to start is with your e-mail communication opens and click-throughs, data that your e-mail marketing service provider makes available to you.

3. Determine how to divide your e-mail list.
When it comes to who you will send your test to, you have two options. You can either split your entire list in half and test one half against the other or take a random sample and do a pre-test.

A pre-test is an excellent way to find out what works before sending the e-mail to your entire list. This knowledge can greatly improve your overall response rate. It also protects you from sending a poor e-mail test to a large portion of your list--and wasting your efforts. To pre-test, choose a random sampling of 100 people from your master list, then split that in half and send each half one of the two test campaigns.

4. Test, measure, and declare a winner.
Once you have everything ready, send your test e-mails. The great thing about e-mail is that you get your results quickly: within a 24-to-48-hour period you will know which e-mail communication got a better result. Declare your winner, send that e-mail to the remaining members of your list and watch the results come in.

5. Have fun and keep it up.
Did I mention that testing is fun? Make a guess about which version will win before you send and see if you are right. What's amazing about testing--and what proves its incredible value--is that many times the results are not at all what you expect.

Let your customers, clients or members tell you, through their actions, what they respond to best. This is an excellent and trustworthy way to improve your e-mails. Test often. You may be surprised every time.